Fracture roughness and gouge distribution of a granite shear band

Abstract

[1] Localization of deformation during fracture mechanical tests leads to the development of shear bands. We performed triaxial tests using Sidobre granite at four different confining pressures (from 20 to 80 MPa). We compared two sets of tests: one set was stopped immediately after the formation of the shear band; a second one included additional shear deformation. From the analysis of thin sections of these laboratory samples, we characterize the typical microstructures in the shear band (mode I and II cracks, Riedel cracks, cataclastic flow). Statistical properties of rupture surface roughness and gouge grain size reveals scaling invariance. Using a mechanical profiler, the fracture roughness is measured along parallel profiles and shown to be correctly described over up to 3 orders of magnitude by self-affine geometry with a roughness exponent close to z = 0.80. This property is very similar to tensile crack even if local processes are different. The influence of the slip is observed. Fracture surfaces are rougher along the slip direction (z = 0.74) than perpendicular to it (z* = 0.80). The confining pressure is shown to have a weak effect on the fracture roughness. It smoothes the surface: slight increase of the roughness exponent. Gouge particles extracted from the shear band present a power law distribution with an exponent ranging from 1.44 to 1.91. This exponent appears to increase with the shearing displacement and the confining pressure. When a significant shear of the band is combined with a high confining pressure (i.e., impeded dilation of the band), the hallmark of fragmentation is observed for the particle distribution and related to a smoothing of the band boundary.

Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

“…We used a cohesive strength of 35 MPa as representative of granitoids (Amitrano and Schmittbuhl, 2002), with a resulting tensile strength of 17.5 MPa.…”

Comportement hydraulique des milieux faillés /

“…Les virgations de grande taille entraînent typiquement la formation de conduits de dilatation avec bréchification (rupture en tension) et augmentation notoire de Km-Ce type d'objet est très bien documenté en géologie structurale et reconnu sous le nom de virgation extensive (extensive jog), par opposition aux virgations compressives dirigées contre le sens de glissement sur la faille. D'autre part, Amitrano et Schmittbuhl (2002) , jusqu'à former des conduits longitudinaux (figure 1.4-b). de Caine et al (1996) sont très largement représentés dans ia nature.…”

“…II est souvent avancé que la topographie des parois des discontinuités naturelles de toutes dimensions peut être ajustée par une fonction fractale auto-afïine sur au moins trois ordres de grandeurs (e.g., Poon et al, 1992 ;Schmittbuhl et al, 1995 ;Amitrano et Schmittbuhl, 2002). Schmittbuhl et al (1995) (Power et al, 1987 ;Schmittbuhl et al, 1993) et sur la fracturation associée à ces failles (Amitrano et Schmittbuhl, 2002).…”

“…Schmittbuhl et al (1995) (Power et al, 1987 ;Schmittbuhl et al, 1993) et sur la fracturation associée à ces failles (Amitrano et Schmittbuhl, 2002). Amitrano et Schmittbuhl (2002) observent expérimentalement que la rugosité des fractures orientées parallèlement au glissement de la faille est supérieure à celles orientées perpendiculairement, impliquant une anisotropie des propriétés hydrauliques macroscopique tangentielles au plan de faille.…”

Relation between first arrival time and permeability in self-affine fractures with areas in contact

“…Self-affine fractures are characterized by a scaling invariance of the statistical properties of the surface roughness under a rescaling of the distances by a factor λ in the average fracture plane and a rescaling λ ζ of the heights. Here ζ is the Hurst or roughness exponent which takes value close to 0.8 for rocks such as granite [20] and values close to 0.5 for porous rocks like sandstone [21,22].…”